Factors Affecting Microbial Growth in Food PDF

Chapter two Factors that affect and favor microbial growth in foods 11/22/2024 1  Outline  Objective  Requirement for growth of microorganisms  Parameters of food that affect microbial growth  Intrinsic factor  Extrinsic fac...

Chapter two Factors that affect and favor microbial growth in foods 11/22/2024 1  Outline  Objective  Requirement for growth of microorganisms  Parameters of food that affect microbial growth  Intrinsic factor  Extrinsic factor  Summary 11/22/2024 2  Objective  At the end of this chapter, the student should be able to :  Explain the intrinsic and extrinsic factors that affect and favor microbial growth in food. 11/22/2024 3  Intrinsic and Extrinsic factors that affect microbial growth in food  Requirements for growth of microorganisms Nutrients Water Carbon, Nitrogen Organic growth factors Minerals Favorable environment Temperature PH Gas mixture No inhibitors Time 11/22/2024 4 What parameters of foods affect microbial growth?  Intrinsic parameters: pH, Moisture content, Oxidation-reduction potential, Nutrient content, Antimicrobial constituents (inhibitors), Biological structures.  Extrinsic parameters: Temperature of storage, Relative humidity, Gaseous environment. 11/22/2024 5  I. The intrinsic factors  These are inherent parts of the plant or animal tissues. It includes:  PH:  Measure of hydrogen ion concentration (-log[H+])  Most organisms grow best around pH 7.0  Bacteria have narrowest pH range, then yeasts, then molds  Pathogens tend to have narrowest pH range  Organic acids are more inhibitory than inorganic acids 11/22/2024 6  Intrinsic factors Cont’d…  Foods usually in acid →neutral pH range:  Fruits →Vegetables →Meats, Milk  Buffering capacity of food reflects:  Resistance to pH change, rate of pH change  Effects of acids (that are less than the minimum required for growth) on organisms Prevent energy production required to maintain cell's internal pH, Enzyme activity affected, Proteins, DNA, other molecules denatured Longer lag, less rapid growth. 11/22/2024 7 11/22/2024 8  Intrinsic factors Cont’d…  Some foods, fruits, soft drinks, vinegar and wines fall below the point at which bacteria normally grow.  Fruits generally undergoes mold and yeast spoilage and  This is due to the capacity of these organisms to grow at pH values < 3.5  Considerably below the minimum pH for most food spoilage and all food poisoning bacteria.  Most meats and sea foods have a final pH of about 5.6 and above  Make these products susceptible to bacteria, mold and yeast spoilage. 9  Intrinsic factors Cont’d…  With respect to the keeping quality of meats,  Meat from fatigued animals spoils faster than from rested animals,  That is a direct consequence of final pH attained up on completion of rigor mortis.  Up on the death of a well rested meat animals,  The usual 1% glycogen is converted into lactic acids which directly causes a reduction in pH values from about 7.4 to 5.6,  This could vary from animals to animals.  The PH values of beef after rigor mortis is thought to be 5.1-6.2. 11/22/2024 10 11/22/2024 11  Intrinsic factors Cont’d…  Regarding to fish, which attains a pH of 5.6 has better keeping qualities than fish with pH of 6.2-6.6.  Some foods are characterized by inherent acidity, others obtain their acidity, due to the action of certain microorganisms which is called biological acidity.  This type of acidity is observed in fermented milk and other foods.  The inherent acidity of foods may be thought as  Natural way of protecting the respective plants or animals tissues from destruction by microorganism. 11/22/2024 12  Intrinsic factors Cont’d…  It is of interest that, fruits should have pH values below those required by many spoilage microorganisms.  The biological function of the fruits is the protection of the plant’s reproduction body, the seed.  The pH of the living animal favors the growth of most spoilage organisms,  Other intrinsic parameters come into play, to permit the survival and growth of microorganism in the animal. 11/22/2024 13  Intrinsic factors Cont’d…  Foods that able to resist change in pH than others are called buffered foods. E.g. Meat is highly buffered than vegetables due to low protein content of vegetables that lack buffering capacity to resist changes in pH by the growth of microorganism. 11/22/2024 14  What are the effects of pH on microbial cells?  Effect on enzymatic activity,  Effect on transportation of nutrients into the cells,  The amino acid decarboxylase that have optimum pH of 4.0,  No activity at pH of 5.5  Bacteria such as Clostridium acetobutylicum,  Raises pH by reducing substrates butyric acid to buthanol.  Enterobacter aerogenes produces acetoin from pyruvic acid to raise the pH of it’s growth environment.  When amino acids are decarboxylated, the increase in pH occurs from the resulting process. 11/22/2024 15  Effects of pH on microbial cells Cont’d…  With respect to the transport of nutrients, the bacterial cells tend to have a residual negative charge.  Nonionic compound therefore can enter cells, but not ionized one,  While at acid pH values, compounds are non-ionized and can enter the negatively charged cells.  If ionic characteristics is affected, it result in denaturation of membranes and transport enzymes. 11/22/2024 16  Effects of pH on microbial cells Cont’d…  The morphology of some microorganisms could be changed by change in pH,  E.g. The length of hyphae of Penicillium chrysogenum could be decreased if the pH values are above 6.0.  Hydrogen ion may favor fermentation process while potassium ion could suppress.  The metabolism of glucose in yeast cells, in acidic medium markedly stimulated by high concentration of potassium ion. 11/22/2024 17  Effects of pH on microbial cells Cont’d…  With respect to temperature, pH of substrate becomes more acidic as temperature increases.  Concentration of salts has a definite effect on pH growth rate curves  Where the addition of 0.2 M Sodium chloride broadened the pH growth range of Alcaligenes.  An adverse pH make cells more sensitive to toxic agents and  Young cells are more susceptible than older cell  The length of lag phase could be increased 11/22/2024 18  Effect of Moisture content for microbial growth in foods  Desiccations or drying are one of the oldest means of preserving foods.  It is due to a direct consequence of removal of moisture, without which microorganism can not grow.  The water activity of the microorganisms is defined by the water activity in the environment (Aw).  It is defined by the ratio of water vapor pressure of food substrate (p) to the vapor pressure of pure water (po), at the same temperature. Aw = P/ Po = solute/solvent 11/22/2024 19  Moisture content cont’d…  Where, P-Vapor pressure of solution, Po-vapor pressure of solvent usually water.  This idea is related to relative humidity (RH), RH= 100 X Aw  Pure water has an Aw of 1.00;  22% NaCl solution has an Aw of 0.86  Saturated solution of NaCl has an Aw of 0.75. 11/22/2024 20  Moisture content cont’d…  The Aw of most fresh foods is above 0.99.  Bacteria require higher values of Aw for growth than fungi.  gr. negative bacteria has higher requirements of Aw than gr. positive.  Most spoilage bacteria do not grow 0.91; pathogens > 0.95  Various solutes used to tie up water (used as preservative)  Degree of inhibition: salt > sugar > glycerol 11/22/2024 23  Water activity Cont’d…  Organisms tolerant of low Aw:  Halophilic bacteria, osmophilic yeasts and xerophilic molds  Halotolerant - able to grow in the presence of high concentrations of salt. – Bacillus, Micrococcus, Staphylococcus, Pediococcus, Vibrio, and Corynebacterium are included in this group.  Osmotolerant - able to grow in the presence of high concentrations of unionized organic compounds such as sugars. Some species from genera Staphylococcus, Leuconostoc, and Lactobacillus are included in this group.  Xerotolerant - able to grow on dry foods. 24  Oxidation-reduction potential (Eh)  Oxidation = loss of electrons; reduction = gain of electrons  O-R potential = electrons are lost or gained  Potential reflects the availability of free oxygen Low potential = anaerobic; high potential = aerobic  On the basis of their growth in the presence and absence of free oxygen,  Microorganisms have been grouped as aerobes, anaerobes, facultative anaerobes, or microaerophiles.  Ranges for microorganisms: Molds are usually aerobic (strict) Yeasts are usually aerobic or facultative Bacteria from strict anaerobes to strict aerobes 11/22/2024 25  O-R potential Cont’d…  O-R potential in food is determined by:  Chemical composition of food,  Oxygen tension and access of atmosphere, Oxygen can be present in a food in the gaseous state (on the surface, trapped inside) or in dissolved form Food stored in air will have a higher redox potential, than stored under vacuum or in modified gas (such as CO2 or N2). Low Oxygen tension- inhibits strict aerobes and delay growth of facultative anaerobes, Obligate anaerobes grow at low or negative redox potentials – require oxygen to be absent.  Natural reducing agents in foods:  SH group of proteins, ascorbic acid, reducing sugars (low pH → high redox-potential (Eh)) 26  Nutrient Content  Nutrients include: carbohydrates, proteins, lipids, minerals, and vitamins.  Water is not considered a nutrient, but it is essential as a medium for biochemical reactions vital for the synthesis of cell mass and energy  Organic growth factors - needed to varying degrees:  Gram (+) > Gram (-) > yeasts >molds  How easily are energy sources metabolized?  Sugar > alcohol > amino acids > complex molecules  How easily are nitrogen sources metabolized?  Amino acids > Proteins  B-vitamins are required by many bacteria  E.g. Enterococcus spp. require B vitamins for growth 11/22/2024 27  Antimicrobial constituents  Foods contain many chemicals, either naturally or added, that adversely affect microbial growth.  Some of the natural inhibitors in foods are:  Spices: eugenol in cloves  Milk: lactoferrin, agglutinine, lactoperoxidase  Eggs: lysozyme  Fruits: organic acids, hydrocinnamic acid derivatives  The inhibitors, depending on their mode of action,  Prevent or reduce growth of and kill microorganisms. 11/22/2024 28  Biological structures  Protective structures (primary defense)  Skin of fruits (tough-protection for seeds)  Shells of eggs, nuts 11/22/2024 29  II. Extrinsic parameters  Temperature  Growth rate vs. Temperature:  Rate is highest at optimum temperature  Growth slows but may not stop at freezing point  Rate falls sharply at temperatures above optimum  Survival vs. temperature:  Assume organisms survive freezing  Organisms killed by exposure to high temperatures  Temperature range for best growth: 15 - 45 C 11/22/2024 30  Temperature Cont’d…  Groupings based on preferred temperature range:  Psychrophiles vs. Psychrotrophs  Mesophiles  Thermopiles vs. Thermoduric  Storage temperature is key factor in stability of perishable foods. 11/22/2024 31  Basic temperatures for microbial growth Temperature 0c Group Minimum Optimum Maximum Thermophile 40-45 55-75 60-90 Mesophile 5-15 30-40 40-47 Psychrophiles (obligate -5 to +5 12-15 15-20 Psychrophiles) Psychrotrophs (facultative -5 to +5 25-30 30-35 psychrophiles) 11/22/2024 32  Relative humidity (RH)  Is a measure of the water activity of the gas phase.  Temperature usually inversely proportional to RH  Surface spoilage retard is by low RH  The storage of fresh fruit and vegetables requires very careful control of RH.  If it is too low then many vegetables will lose water and become flaccid.  If it is too high then condensation may occur and microbial spoilage may be initiated. 11/22/2024 33  Gases in the environment  Controlled or modified atmosphere storage:  Carbon dioxide is included for its inhibitory effect  Increased CO2 retard fungal rot in fruits NOTE  Growth rate: Bacteria > yeasts > molds  Optimum to stress condition: Bacteria > yeasts > mold  Pathogens – tolerate stress poorly 11/22/2024 34 Summary Organisms require nutrient, favorable environment and time to grow. Two factors which affect and favor microbial growth in food  Intrinsic factors  pH, moisture content, oxidation-reduction potential, nutrient content, antimicrobial constituents, biological structures.  Extrinsic factors  Temperature of storage, relative humidity, gases in the environment. 11/22/2024 35